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Incorporating Zwitterionic Graphene Oxides into Sodium AlginateMembrane for Efficient Water/Alcohol SeparationJing Zhao,†,‡ Yiwei Zhu,†,‡ Guangwei He,†,‡ Ruisi Xing,†,‡ Fusheng Pan,†,‡ Zhongyi Jiang,*,†,‡

Peng Zhang,§,∥ Xingzhong Cao,§,∥ and Baoyi Wang§,∥

†Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, TianjinUniversity, Tianjin 300072, China‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China§Multi-discipline Research Division, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China∥Beijing Engineering Research Center of Radiographic Techniques and Equipment, Beijing 100049, China

*S Supporting Information

ABSTRACT: For the selective water-permeation across densemembrane, constructing continuous pathways with high-density ionic groups are of critical significance for thepreferential sorption and diffusion of water molecules. In thisstudy, zwitterionic graphene oxides (PSBMA@GO) nano-sheets were prepared and incorporated into sodium alginate(SA) membrane for efficient water permeation and water/alcohol separation. The two-dimensional GO providescontinuous pathway, while the high-density zwitterionicgroups on GO confer electrostatic interaction sites withwater molecules, leading to high water affinity and ethanolrepellency. The simultaneous optimization of the physical andchemical structures of water transport pathway on zwitterionic GO surface endows the membrane with high-efficiency waterpermeation. Using dehydration of water/alcohol mixture as the model system, the nanohybrid membranes incorporatingPSBMA@GO exhibit much higher separation performance than the SA membrane and the nanohybrid membrane utilizingunmodified GO as filler (with the optimal permeation flux of 2140 g m−2 h−1, and separation factor of 1370). The study indicatesthe great application potential of zwitterionic graphene materials in dense water-permeation membranes and provides a facileapproach to constructing efficient water transport pathway in membrane.

KEYWORDS: zwitterionic, graphene oxides, nanohybrid membrane, water transport pathway, water/alcohol separation

■ INTRODUCTION

The selective water-permeation across dense membrane is apivotal issue in various membrane processes includingpervaporation, vapor permeation, and reverse osmosis, withapplications spanning from bioalcohol recovery to watertreatment.1−3 Hydrophilic groups are imperative carriers forwater permeation, which comprise ionic/nonionic types, andbind water molecules via electrostatic interaction/hydrogenbond, respectively.4 Because of the higher bond strength andlower directionality of electrostatic interaction compared withhydrogen bond, the ionic groups are usually conferred withhigher water affinity and water uptake,5 which are favorable forthe preferential sorption of water molecules, the initial step ofwater permeation process. Apart from the type of hydrophilicgroups, the group density and distribution are also criticalfactors for water permeation, because they determine theproximity effect of the neighboring hydrophilic groups and theconnectivity of the water transport pathway, exerting greatinfluence on water diffusion,6 another step of water permeationprocess. Therefore, constructing continuous pathway with high-

density ionic groups in membrane is a viable strategy to acquirehigh-efficiency water permeation.Recently, zwitterionic materials possessing moieties with

both cationic and anionic groups have emerged as promisingsuperhydrophilic materials.7−9 Compared with other hydro-philic materials, zwitterionic material has the followingadvantages: (i) the abundant ionic groups provide affluentelectrostatic interaction sites with water molecules, thusinducing denser and stronger hydration layer, and leading tohigher repellency against competitive molecules;5,8,10 (ii) thewater molecules outside the hydration layer of zwitterionicmaterials possess higher freedom compared with those outsidethe hydration layer induced by hydrogen bond;5 (iii) thedistinct charge-neutral feature endows zwitterionic monomerswith higher reactivity for free radical polymerization comparedwith other charged monomers because of the increased affinity

Received: November 2, 2015Accepted: January 5, 2016Published: January 14, 2016

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between monomers, thus easily obtaining polyzwitterions withhigh ionic group density.11,12 All the above features are in favorof the selective water-permeation across membrane. Up to now,a vast number of zwitterionic materials have been exploitedwith their hydration properties being intensively studied.However, the applications of zwitterionic materials inmembrane field mainly focus on the construction of super-hydrophilic membrane surface to form hydration layer andeliminate fouling in porous membranes.13−16 Their effects onselective water-permeation in dense membranes have rarelybeen investigated.In this study, sulfobetaine methacrylate (SBMA) was utilized

as the monomer to prepare zwitterionic graphene oxide (GO)nanosheets with high grafting density via free radical polymer-ization method. SBMA is a representative zwitterionic materialpossessing quaternary ammonium group with positive chargeand sulfonate group with negative charge simultaneously, whichare both completely dissociated in water,17 thus generatingabundant electrostatic interaction sites with water molecules.GO is a burgeoning two-dimensional carbon material with one-atom thickness, high aspect ratio, high specific surface area andrich modification approaches.18 Consequently, the nanohybridmembranes with GO-based fillers can confer continuoustransport pathways with chemical diversity on GO surfaces.19,20

Herein, polymer-inorganic nanohybrid membranes werefabricated by incorporating zwitterionic GO into sodiumalginate (SA) matrix (Figure 1) and utilized for water/alcohol

separation. The effects of incorporating continuous zwitterionicpathway on nanohybrid membrane structure and membraneseparation performance were investigated.

■ MATERIALS AND METHODSMaterials. Natural flake graphite (2500 mesh) was received from

Qingdao Tianhe Graphite Co. Ltd. (Shandong, China). Hydrochloricacid (HCl, 36−38 wt %), potassium permanganate (KMnO4) andsulfuric acid (H2SO4, 98 wt %) were purchased from Tianjin KeweiLtd. (Tianjin, China). Sodium nitrate (NaNO3), hydrogen peroxideaqueous solution (H2O2, 30 wt %), ethanol (≥99.7 wt %), N,N-dimethylformamide (DMF, ≥ 99.9 wt %), sodium hydroxide (NaOH),and calcium chloride (CaCl2) were supplied by Tianjin Guangfu FineChemical Research Institute (Tianjin, China). Ammonium perox-ydisulfate ((NH4)2S2O8, 99.99 wt %) was supplied by ShanghaiAladdin Biochemical Technology Co., Ltd. (Shanghai, China).Sulfobetaine methacrylate (97 wt %) was purchased from Sigma-Aldrich (USA). Sodium alginate (SA) was received from QingdaoBright Moon Seaweed Group Co. Ltd. (Shandong, China). Thepolyacrylonitrile (PAN) ultrafiltration membrane (molecular weightcutoff: 100 kDa) with the flat-sheet configuration was purchased fromShandong MegaVision Membrane Technology & Engineering Co.Ltd. (Shandong, China). All chemicals used were in analytical grade.Deionized water was utilized in all of the experiments.

Preparation of Zwitterionic Graphene Oxides. Zwitterionicgraphene oxides were prepared via free radical polymerizationmethod.21,22 First, GO dispersion was prepared following the modifiedHummers method as described in our previous study.23 Afterward, 40mL of GO dispersion (2.5 mg mL−1 with water as solvent) and 90 mLof DMF were added into a flask and sonicated for 30 min. Then, 50mL of SBMA aqueous solution (60 mg mL−1) was added into the GOdispersion and stirred for 30 min with the flask in a thermostated oilbath at 60 °C. One hundred milliliters of (NH4)2S2O8 aqueoussolution (1.2 mg mL−1) was prepared and added dropwise through aconstant pressure funnel after the solution was purged under nitrogenfor 30 min. The reaction mixture was stirred at 60 °C under nitrogenfor 40 h. After that, the mixture was cooled to room temperature,diluted with 400 mL of water, sonicated for 1 h, and washed with waterand ethanol. The obtained precipitates after centrifugation werezwitterionic GO, designated as PSBMA@GO.

Fabrication of Nanohybrid Membranes. The nanohybridmembranes were fabricated via spin-coating method. First, theremoval of glycerin from the PAN membrane was achieved byimmersing it in water for 48 h, and then the membrane was fixed onspin coater after fully dried. Meanwhile, PSBMA@GO aqueousdispersion was prepared and sonicated for 1 h. After sonication, SAwas added and stirred for 4 h at 30 °C to obtain a homogeneous SA-PSBMA@GO casting solution. The SA concentrations were 1.5 wt %for all the membranes, while the mass ratio of PSBMA@GO to SAvaried from 0 wt % to 7.5 wt %. After filtrated and kept still for severalminutes to remove air bubbles, the SA-PSBMA@GO casting solutionswere spin-coated on PAN ultrafiltration membranes and dried at roomtemperature. Finally, the as-fabricated nanohybrid membranes weresoaked in a 0.5 M CaCl2 solution for 10 min, then rinsed withdeionized water and dried at room temperature. The obtainedmembranes were named as SA-PSBMA@GO(X)/PAN, in which Xwas the mass ratio of PSBMA@GO to SA. Meanwhile, thehomogeneous membranes were prepared for characterization bycasting the same solutions on glass plates and named as SA-PSBMA@GO(X). For comparison, the membrane with pristine GO as filler wasfabricated and named as SA-GO(Y)/PAN, in which Y was the GO/SAmass ratio.

Membrane Characterizations. The morphology of PSBMA@GO was observed by transmission electron microscopy (TEM, JEOLJEM-100CXII) and atomic force microscope (AFM, CSPM 5000).Element mapping equipped on TEM was employed to characterize theelement distribution on PSBMA@GO. FTIR spectra with a scan rangeof 2500−450 cm−1 were measured using BRUKER Vertex 70 FT-IRspectrometer. The chemical compositions of pristine and zwitterionicGO were measured by X-ray photoelectron spectroscopy (XPS, KratosAxis Ultra DLD) with a monochromatic Al Kα source and a chargeneutralizer to ascertain the successful grafting of PSBMA. Themeasurement of static water contact angles was performed using acontact angle goniometer (JC2000C Contact Angle Meter) to evaluatethe hydrophilicity of GO and PSBMA@GO. Field emission scanningelectron microscope (FESEM) (Nanosem 430) was utilized to observethe cross-section and surface morphologies of membranes. To acquirethe thermal properties of GO and PSBMA@GO, thermogravimetricanalysis (TGA, NETZSCH TG 209 F3) was used heating from 40 to800 °C (rate = 10 °C min−1) in N2 atmosphere. The free volumeproperties of membranes was analyzed by positron annihilationlifetime spectroscopy (PALS) using an EG&G ORTEC fast−slowcoincidence system with a resolution of 208 ps. The positron source(22Na, 13 μci) was sandwiched between two pieces of samples withsize of 1 cm × 1 cm and thickness of approximately 0.5 mm. Asemiempirical equation given by eq 1 correlates o-Ps annihilationlifetime (τ3 and τ4) with the radius of the free volume cavity, r3 and r4(Δr = 0.1656 nm).24

τπ

π= −+ Δ

++ Δ

−⎜ ⎟⎜ ⎟

⎡⎣⎢

⎛⎝

⎞⎠

⎛⎝

⎞⎠⎤⎦⎥

rr r

rr r

12

11

2sin

21

(1)

Figure 1. (a) Schematic representation of the membrane fabricationprocedure and (b) molecular structure of zwitterionic monomersulfobetaine methacrylate.

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The apparent fractional free volume ( fapp) of membranes could becalculated with the following equation (I is the annihilation intensity ofo-Ps):

π= +f r I r I43

( )app 33

3 43

4 (2)

Membrane Separation Experiments. The pervaporationseparation of water/ethanol mixture was utilized to evaluate theperformance of nanohybrid membranes for selective water-perme-ation.Pervaporation experiments were performed with a P-28 membrane

module (CM-Celfa AG Company, Switzerland).3 The downstreampressure of membrane was kept below 0.3 kPa with a vacuum pump,and the flow rate of feed solution was controlled at 60 L h−1. Thepermeate was collected with a liquid nitrogen cold trap after themembrane reached steady state. The permeate solution was weighed.Gas chromatography (Agilent 7820A, USA) was utilized to analyze thecompositions of feed and permeate solutions. The permeation flux (J,g m−2 h−1)), separation factor (α) and pervaporation separation index(PSI) of the membranes are calculated through the equations asfollows:

=×

JQ

A t (3)

α =P PF F

//

W E

W E (4)

α= × −JPSI ( 1) (5)

where Q represents the mass of collected permeate (g), t representsthe collection interval (0.5 h), and A represents the actual membranearea contacting with feed (2.56 × 10−3 m2). P represents the massfraction of water (subscript W) or ethanol (subscript E) in permeatesolution, while F represents the mass fraction of water or ethanol infeed solution. Three membranes were prepared under every condition,and each membrane was measured repeatedly for three times to ensurethe reliability of experimental data. The average of multiple results wasutilized as the final data.

■ RESULTS AND DISCUSSIONMorphology and Chemical Structure of PSBMA@GO.

The morphology of PSBMA@GO was characterized by TEMand AFM. Compared Figure 2a and b, it can be observed thatthe transparent and lamellar morphology of GO is preservedafter modification, while the transparency decreases because ofthe covering of PSBMA on GO.25 Figure 2c−f show thedistribution of C, N, and S elements on PSBMA@GO, amongwhich N and S are the characteristic elements of SBMAmonomer. Therefore, Figure 2e and f confirm the successfulgrafting and uniform distribution of PSBMA on GO. The 20μm × 20 μm AFM image in Figure 2g exhibits the irregularshape and submicrometer-scale size along horizontal directionof PSBMA@GO. The thickness of PSBMA@GO sheets ismeasured to be about 4.1 nm from the height profile in Figure2h, which is much higher than that of GO, furtherdemonstrating the success of polymer grafting.22

The FT-IR spectra of pristine and zwitterionic GO wereobtained as shown in Figure 3a to analyze the chemicalstructure transformation of GO after modification. Several newadsorption peaks appear on the spectrum of PSBMA@GO at1720 cm−1 because of the stretching vibration of CO inester group, 1170 cm−1 due to the asymmetric stretchingvibration of OSO in sulfonate, and 1036 cm−1 due to thestretching vibration of CN in quaternary ammoniumgroup, corresponding to the chemical groups on SBMA.12,15

XPS analysis of GO and PSBMA@GO was performed to obtain

the grafting amount of PSBMA. As shown in Figure 3c, N 1s, S2s, and S 2p peaks appear on the spectrum of PSBMA@GO.According to the S element content of 7.66 wt %, the contentof PSBMA in PSBMA@GO can be calculated to be 66.79 wt %.Therefore, the grafting amount of PSBMA is 2.01 g g−1, whilethe grafting density is 1.67 SBMA unit nm−2, which means thatthere are 3.34 charged groups in the area of one squarenanometer. The hydrophilicity of GO and PSBMA@GO wascharacterized by measuring the water contact angles (Figure3d). The employed samples are GO and PSBMA@GOmembranes fabricated via suction filtration of the correspond-ing aqueous dispersions. The water contact angle declines from47.3 o to 29.4 o after zwitterionic modification of GO, revealingthe significantly improved hydrophilicity, which arises fromthree reasons. First, the grafting of PSBMA on GO isperformed via free radical polymerization, which occurs onthe carbon−carbon double bonds of GO instead of the oxygen-containing groups commonly used in other chemicalmodification.21,22 Therefore, the original hydrophilic groupson GO are reserved. Second, zwitterionic unit possessesabundant charged groups, which bind water molecules throughstrong electrostatic interaction, while most of the hydrophilicgroups on GO are neutral and bind water molecules throughhydrogen bonds. The water molecules adsorbed throughelectrostatic force are less direction sensitive than thosethrough hydrogen bond. It has been reported that abouteight water molecules can be tightly bound with one SBMAunit.5 Last but not least, the grafting amount of PSBMA ismuch higher than the original hydrophilic groups on GO. Inorder to investigate the change of GO’s affinity for water andethanol after modification, both GO and PSBMA@GOdispersions with water and ethanol as solvents were prepared.As shown in Figure S1, GO disperses well both in water andethanol, while PSBMA@GO precipitates in ethanol, confirmingits ethanol repellency arising from the abundant charged groupson PSBMA.

Morphology of Nanohybrid Membranes. According tothe grafting amount of PSBMA, the content of GO in SA-

Figure 2. (a) TEM image of GO, (b) TEM image of PSBMA@GO,(c−f) element mapping of PSBMA@GO, (g) AFM image ofPSBMA@GO, and (h) height profile of PSBMA@GO.

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m.cnPSBMA@GO(2.5)/PAN membrane is calculated to be about0.8 wt %. Therefore, the SA-GO(0.8)/PAN and SA-GO(0.8)membranes were fabricated for comparison. The cross-sectionmorphologies of SA control membrane and nanohybridmembranes were observed via SEM. It is shown in Figure 4athat the separation layer with the thickness of 232 ± 12 nm istightly adhered to the PAN support layer. The SA controlmembrane exhibits smooth cross-section morphology (Figure4b), while brick-and-mortar architectures appear after incorpo-rating PSBMA@GO or GO (Figure 4c−f). The PSBMA@GOand GO nanosheets orient randomly in SA matrix under lowcontent (Figure 4c and f). With the increase of loading content,more PSBMA@GO nanosheets are prone to aligning parallel tothe membrane surface (Figure 4d and e). Both PSBMA@GOand GO were dispersed uniformly without apparent agglom-eration in all the membranes.Physical Structure of Nanohybrid Membranes. The

free volume properties of SA control membrane and nano-hybrid membranes were characterized by PALS as shown inTable 1. r3 and I3 are the radius and intensity of network pores,which mean the small cavities between polymer chains. r4 and I4

are the radius and intensity of aggregate pores, including thelarger spaces surrounding the polymer aggregates and thespaces at polymer-inorganic interface.26 With the augment ofPSBMA@GO content, both the radiuses of network pore andaggregate pore slightly diminish and then remarkably increase,while the intensities almost decrease all along. As a result, thefractional free volume ( fapp) fluctuates in a narrow range. Afterincorporating PSBMA@GO, the electrostatic interactionsbetween carboxyl groups on SA and quaternary ammoniumgroup on SBMA restrict the mobility of SA chains, conferringthe membrane with more compact structure and smaller freevolume cavities. Meanwhile, the appearance of polymer−inorganic interface increases the aggregate pore intensity. Athigher PSBMA@GO content, the restacking of nanosheets mayoccur, which brings multiple influences to the free volumeproperties of the polymer matrix, polymer-inorganic interface,and inorganic filler itself: (i) the interface area and interfacialinteraction sites does not increase proportionally withPSBMA@GO content, leading to the reduced influence onpolymer chains mobility; (ii) the two-dimensional PSBMA@GO can act as nucleating agent to induce the orderedarrangement of polymer chains on the surface of nanosheets,leading to the increased crystallinity in interface region; (iii)

Figure 3. (a) FT-IR spectra of GO and PSBMA@GO, (b) XPSspectrum of GO, (c) XPS spectrum of PSBMA@GO, and (d) watercontact angles of GO and PSBMA@GO.

Figure 4. SEM images of the cross-section morphologies of (a) SA-PSBMA@GO(2.5)/PAN, (b) SA, (c) SA-PSBMA@GO(2.5), (d) SA-PSBMA@GO(5), (e) SA-PSBMA@GO(7.5), and (f) SA-GO(0.8).

Table 1. Free Volume Properties of SA Control Membraneand Nanohybrid Membranes

membrane r3 (nm) I3 (%) r4 (nm) I4 (%) fapp

SA 0.129 13.31 0.287 5.59 0.215SA-PSBMA@GO(2.5) 0.128 12.80 0.282 5.74 0.208SA-PSBMA@GO(5) 0.147 11.78 0.305 4.19 0.210SA-PSBMA@GO(7.5) 0.203 9.76 0.340 2.12 0.220SA-GO(0.8) 0.195 11.07 0.332 2.47 0.229

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PSBMA@GO may bring free volume cavities betweennanosheets. Compared with SA-GO(0.8) membrane, the SA-PSBMA@GO(2.5) membrane possesses free volume cavitieswith smaller size and higher intensity, arising from the differentinterfacial chemical compositions and interactions.Separation Performance of Nanohybrid Membranes.

To investigate the effect of incorporating PSBMA@GO onselective water-permeation, the pervaporation experiments ofSA control membrane and nanohybrid membranes wereperformed with 90 wt % ethanol aqueous solution at 350 K.As shown in Figure 5a, both the permeation flux and separation

factor of SA-PSBMA@GO/PAN membrane reach the max-imum (with the permeation flux of 2140 g m−2 h−1 and theseparation factor of 1370) when the content of PSBMA@GO is2.5 wt %. Because the size of free volume cavities in SA-PSBMA@GO(2.5)/PAN membrane is similar to that of SAcontrol membrane (Table 1), the influence of free volume onseparation performance is negligible. Therefore, the enhance-ment of permeation flux and separation factor afterincorporating PSBMA@GO is attributed to the introductionof zwitterionic pathway. The dense quaternary ammonium andsulfonate groups on PSBMA@GO surface bind abundant watermolecules and then form hydration layer, which preferentiallyinteracts with water molecules in feed solution and repelsethanol molecules, thus generating a specific and efficient watertransport pathway on PSBMA@GO surface. As a result, thewater flux augments while ethanol flux exhibits a reversetendency when increasing the PSBMA@GO content as shownin Figure 5b. With the further increase of PSBMA@GOcontent, more PSBMA@GO nanosheets are prone to aligningparallel to the membrane surface, which prolongs the diffusionpathway of water and ethanol molecules. Meanwhile, theincrescent size of free volume cavities weakens the molecularsieving effect for ethanol molecules. Consequently, the water

flux declines while the ethanol flux almost keeps constant,leading to the decreased permeation flux and separation factorat higher PSBMA@GO content. Compared with SA-GO(0.8)/PAN membrane, higher permeation flux and separation factorare obtained for PSBMA@GO(0.8)/PAN membrane. Thesuperior separation performance can be ascribed to threereasons: (i) the hydration layer on GO surface is inducedmainly via hydrogen bond. Therefore, the water moleculesoutside the hydration layer on PSBMA@GO possess higherfreedom and mobility compared with those outside thehydration layer on GO, which benefits water permeation; (ii)the higher ethanol repellency of PSBMA@GO compared withpristine GO endow the water transport pathway with higherselectivity; (iii) the SA-PSBMA@GO(2.5)/PAN membranepossesses free volume cavities with smaller size, which is infavor of the molecular sieving for ethanol molecules.Figure 6 summarizes the performance of recently reported

water/ethanol separation membranes with different kinds of

materials (such as SA, poly(vinyl alcohol) (PVA), chitosan(CS), and grapheme oxide (GO)). The detailed informationsuch as operation conditions, membrane thickness and PSIvalue is shown in Table S1. Because of the incorporation ofzwitterionic water transport pathway on PSBMA@GO surface,the SA-PSBMA@GO/PAN nanohybrid membrane exhibitshigh permeation flux and desirable separation factor.

■ CONCLUSIONSIn this study, a novel approach to promoting the selectivewater-permeation across dense membrane was explored byconstructing high-efficiency water transport pathway usingzwitterionic GO nanosheets. Zwitterionic GO with highgrafting-density (1.67 SBMA unit nm−2) was prepared viafree radical polymerization, and then incorporated into SAmatrix to fabricate nanohybrid membrane. The membranecontaining 2.5 wt % PSBMA@GO achieves the optimalseparation performance with the permeation flux of 2140 gm−2 h−1 (164% of SA control membrane), and separation factorof 1370 (2.5 times of SA control membrane). Such superiorseparation performance arises from the combination of theoptimized physical structure and chemical structure of watertransport pathway: (i) the two-dimensional GO createspathway with high level of continuity and (ii) the high-densityzwitterionic groups on GO electrostatically induce dense andstrong hydration layer, leading to high water affinity andethanol repellency. Considering the pivotal role of waterpermeation in membrane field, this study may offer a genericand facile approach to preparing highly water-selective

Figure 5. Pervaporation performance of SA control membrane andnanohybrid membranes: (a) permeation flux and separation factor and(b) water flux and ethanol flux.

Figure 6. Comparison of the membrane in this study with recentlyreported membranes in literatures.27−43

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membranes for various dense membrane based separationprocesses.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.5b10551.

Photographs of GO and PSBMA@GO dispersions withwater or ethanol as solvent (Figure S1); TGA curves ofGO and PSBMA@GO (Figure S2); static water contactangles on nanohybrid membrane surfaces (Figure S3);long-term separation performance of SA-PSBMA@GO(2.5)/PAN membrane (Figure S4); comparison ofthe membrane separation performance in this study withrecently reported membranes in literatures (Table S1)(PDF)

■ AUTHOR INFORMATIONCorresponding Author*Fax: +86 22 2350 0086. Tel: +86 22 2350 0086. E-mail:zhyjiang@tju.edu.cn.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors are grateful to the financial support from theNational Natural Science Foundation of China (21490583 and21306131), the National Science Foundation for DistinguishedYoung Scholars (21125627), the Specialized Research Fund forthe Doctoral Program of Higher Education (20120032120009),and the Program of Introducing Talents of Discipline toUniversities (B06006).

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